Glow switch having a cadmium covered electrode



1964 D. T. WAlGH 3,145,280

01.0w SWITCH HAVING A CADMIUM COVERED ELECTRODE Filed Aug. 7, 1959 I Fl,1:- g1 1% w INVENTUR DOUGLAS THQMAS WMG 1+ A TTORNE y United StatesPatent :ULOW swiron HAVING A cAnMiUM covnnnn ELEQTPDEBE Douglas ThomasWaigh, London, England, assignor to Thorn Electrical industries Limited,London, England Filed Aug. 7, 1959, Ser. No. 832,230 Claims priority,application Great Britain Aug. 19, 1958 1 Claim. (Cl. ZOO-113.5)

The present invention relates to glow switches and methods for theirmanufacture.

Such switches are manufactured in large numbers for use as starterswitches for electric discharge tubes, particularly fluorescent lamps.They consist of two electrodes, one or both of which are bi-metallicelements, enclosed in an envelope which is filled with a suitable gas.In operation a glow discharge is established between the electrodes andthe resultant heating causes the electrodes to come into contact withone another.

It is desirable that the operating characteristics of the switch, theglow voltage, glow current, and operating time, should be predictableand should not vary widely during the life of the switch. To this endknown methods of manufacture of glow switches include an ageing processto stabilise the operating characteristics. It is known to plate theelectrodes of the switches with zinc, so as to reduce the time for whichthe switches must be subjected to the ageing process in order to produceacceptable characteristics, and stability of those characteristics, in ahigh proportion of the switches made.

According to the present invention there is provided a method of makinga glow switch, including providing a layer of cadmium on the bi-metallicelectrode or electrodes of the switch, removing most of the air from theswitch, and thereafter evaporating oif some of the cadmium under reducedpressure.

A preferred method of putting the method of the invention into effectwill now be described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a view of a glow switch in elevation, and

FIG. 2 is a partial section on the line Il--II of FIG. 1 showing someparts on an exaggerated scale for greater clarity.

The two bimetallic strips forming the electrodes 1 and 2 are firstplated with a cadmium layer 3 to a thickness of 110% of the thickness ofthe strip. The plated electrodes 1 and 2, which each have the outline ofa parallelogram, are then mounted on respective leads 4 and 5 within aglass envelope 6 so as to lie in parallel spaced planes with their upperends overlapping.

The envelope 6 is evacuated in conventional manner by withdrawing airfrom it and the temperature is simultaneously raised, reaching at least290 C. when the pressure drops to its minimum value. This temperatureensures the removal of water vapour and occluded gases and 3,145,280Patented Aug. 18, 1964 causes evaporation of some of the cadmium layer 3from the electrodes 1 and 2, cadmium vaporising at a lower temperaturethan would zinc in similar circumstances. The clean cadmium surface thusproduced shows less tendency to form an oxide film than a zinc surfacewould in the same conditions and consequently, even though some airremains in the envelope after evacuation, the ageing time for glowswitches produced in the manner described can be reduced to one minute,as compared with five minutes for zinc-plated electrodes, and greaterstability in operation throughout the working life of the switch can beachieved.

After evacuation of the envelope and heating to cause evaporation ofsome cadmium, the envelope is filled with argon and sealed off inconventional manner before the ageing process is carried out.

The range of thicknesses of the cadmium layer given in the exampledescribed is controlled by two factors. As is well known in the art, toogreat a thickness of a thermally inert material such as cadmium willimpair the sensitivity of a bi-metallic strip. While the initialthickness of cadmium can be reduced by evaporation it is not normallyconvenient to have an initial thickness for the cadmium layer greaterthan 10% of the thickness of the bi-metallic strip. The minimum initialthickness is determined by the requirement that some cadmium shall beleft on the electrodes after evaporation, and may be as low as .0005inch Without requiring abnormally strict control of the conditionsduring manufacture.

Where only one electrode is a bi-metallic element it is not necessary toprovide a layer of cadmium on the other, non-bimetallic, electrode.While plating with cadmium offers economic advantages, it is alsopossible to apply the layer of cadmium by other methods, for examplevacuumdeposition.

I claim:

A glow switch comprising a sealed envelope, said envelope beingevacuated of air and filled with an inert gas, two electrodes containedwithin the envelope, and leads for the two electrodes emerging from theenvelope, at least one of the electrodes being a bi-metallic elementhaving a thin layer of cadmium on at least one surface thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,449,236 Lamont Sept. 14, 1948 2,457,487 Peacock et a1. Dec. 28, 19482,488,701 Blood Nov. 22, 1949 2,536,280 Haegele Jan. 2, 1951 2,596,469Cooper May 13, 1952 2,677,071 Carne Apr. 27, 1954 2,740,861 Lake Apr. 3,1956 2,870,524 Kinnear Jan. 27, 1959 2,907,100 Lindblad Oct. 6, 19592,909,777 Germashausen Oct. 20, 1959

